Depression remains a serious health concern that affects approximately 1 in 6 individuals in the U.S. and dramatically decreases the quality of life for those struggling with the illness. However, there is still a great need for improved understanding of the neurobiological processes that mediate the pathogenesis of depression. There is also a need for more effective therapeutic interventions to treat depression. Evidence from clinical and preclinical studies strongly suggests that dopaminergic and cholinergic mechanisms likely play important roles in the pathogenesis of depression. Thus, ongoing investigations have sought to identify the neurocircuitry underlying major depressive disorder (MDD). Recent work in rodent models has revealed a novel, causal role for phasic dopamine activity in the ventral tegmental area (VTA) to nucleus accumbens (NAc) pathway in mediating susceptibility and resilience to stress. However, there remains a critical need to determine whether the mechanisms that regulate phasic dopamine activity also mediate responses to stress. Our preliminary findings demonstrate that VTA muscarinic acetylcholine receptor (mAChR) mechanisms powerfully regulate both phasic DA activity and susceptibility to stress, as revealed through neurochemical and behavioral studies in rats. However, critical gaps remain in identifying both the specific VTA mAChR subtype(s) and the primary cholinergic input into the VTA that mediates this susceptibility. The work in this proposal will use an integrative experimental approach (utilizing behavioral pharmacology, in vivo fast scan cyclic voltammetry, and in vivo optogenetics in male and female Sprague-Dawley rats) to address these gaps in scientific understanding. Behavioral examination will include the use of the chronic unpredictable stress (CUS) model, which has strong construct and face validity as a model of depression. Aim 1 will use behavioral pharmacology and in vivo voltammetry to identify the specific midbrain mAChR subtype(s) that mediate behavioral and dopaminergic responses to stress. Aim 2 will use in vivo optogenetics and behavioral analyses to identify the specific mesopontine to midbrain cholinergic pathway(s) that mediates susceptibility and resilience to chronic stress. The overarching goal of this work is to identify the neurobiological mechanisms that mediate behavioral and physiological responses to stress in order to facilitate the development of novel therapeutic interventions for depression.